In recent years, inks, paints, and coating agents used for image displays or as image-recording agents have been required to exhibit high image quality, high definition image, and high printability, and therefore a pigment dispersion liquid in which a pigment is finely dispersed has been demanded. Specific applications of the pigment dispersion liquid include pigment dispersion liquids for water-based inkjet inks, pigment dispersion liquids for coloring agents used for color filters, pigment dispersion liquids for ultraviolet ray curable type inkjet inks, pigment dispersion liquids for gravure inks, and so on. The pigment dispersion liquid in the present invention means a liquid containing a pigment dispersed at a high concentration in water, an organic solvent, or a polymerizable compound. For example, the pigment dispersion liquid containing a pigment dispersed at a high concentration is used as a coloring agent and is contained in a medium such as water-based medium, oil-based medium, or a resin, thereby making it possible to manufacture a product as a pigment dispersion liquid composition such as an ink or a paint in which the pigment is finely dispersed, the composition containing the pigment dispersion liquid as a constituent component.
The pigment dispersion liquid is used as a coloring agent in the inks and so on, and it is strongly desired that the pigment be highly dispersed and the dispersion state be maintained particularly in the above-listed applications. Therefore, pigment dispersants the structures of which vary from one dispersant to another have been developed, and the pigment dispersant that exhibits a higher degree of fine dispersibility, stability, and fluidity as required performance has been demanded (see, for example, Patent Literatures 1, 2, and 3). Moreover, a pigment dispersant that makes use of an acryl-based polymer has been known as one of these pigment dispersants. And in the methods for producing the acryl-based polymer to be used in such a case, various kinds of living radical polymerization methods have been invented for controlling the structure, however the technology has been applied not only for producing a pigment dispersant but also for producing an adhesive, an elastomer, or the like (for example, Patent Literature 4).